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        <td class="header">&nbsp;Wind Field Estimation</td>


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<h3>Wind Field Estimation</h3>&nbsp;&nbsp;&nbsp;As the wind blows across the ocean
surface, it generates surface roughness generally aligned with the wind
direction. Consequently the radar backscatter from this roughened
surface is related to the wind speed and direction. This operator
retrieves wind speed and direction from C-band SAR imagery. <br><h4>Major Processing Steps</h4>&nbsp;&nbsp;The general
approach for the wind field retrieval is as the follows:<br>
<ol>
    <li>First a land-sea mask is generated to ensure that the estimation&nbsp;is focused only on the sea surface area.
    </li>
    <li>Then the SAR image is divided into grid using user specified window size.</li>
    <li>For
        each grid, a wind direction (with 180� ambiguity) is estimated from
        features in the SAR image using&nbsp;a frequency domain method.
    </li>
    <li>With
        the wind direction estimated for the grid, finally the wind speed is
        estimated by using CMOD5 model for the Normalized Radar Cross Section
        (NRCS).
    </li>
</ol>
&nbsp;&nbsp;For details of land-sea mask generation, the reader is&nbsp;referred to the <a href="CreateLandMask.html">Create
    Land Mask</a> operator.<br><h4>Wind Direction Estimation</h4>&nbsp;&nbsp;
The wind direction is estimated from the features in the SAR image.
Detailed steps for the estimation&nbsp;are given below:<br>
<ol>
    <li>For
        each window within which a wind direction will be estimated, a local
        FFT size is determined. The FFT size is 2/3 of the window size,
        therefore four spectra can be computed in the window with&nbsp;each
        spectra region has a 50% overlap with the neighboring spectrum.
    </li>
    <li>Each window is flattened by applying a large average filter, then dividing by the filtered image.</li>
    <li>The FFT&#8217;s are applied and the four resulting spectra are averaged.</li>
    <li>An
        annulus is applied to the spectrum to zero out any energy outside of a
        wavenumber region. The limits of the annulus are set to wave lengths of
        3 km to 15 km.
    </li>
    <li>A 3x3 median filter is then applied to the spectrum to remove noise.</li>
    <li>A
        2D polynomial is fit to the resulting spectral samples and the
        direction through the origin which has the largest quadratic term (i.e.
        the widest extent) is determined. The wind direction is then assumed to
        be 90 degree from this direction.<br></li>
</ol>
<h4>Wind Speed Estimation</h4>
<ul>
    <li>&nbsp; &nbsp;The wind speed is estimated using the CMOD5 model for NRCS developed by Hersbach <span
            style="font-style: italic;">et al</span>.
        [1]&nbsp;for VV-polarized C-band scatterometry.
    </li>
    <li>&nbsp;&nbsp;&nbsp;For ENVISAT HH-polarized product, where CMOD5 model is not directly
        applicable, the operator first converts the NRCS at HH polarization
        into a corresponding NRCS for VV polarization with the following
        equation, then applies the CMOD5 model to the converted NRCS:
    </li>
</ul>
<br>

<div style="margin-left: 40px;"><img style="width: 255px; height: 67px;" alt=""
                                     src="images/wind_field_estimation_eq_1.jpg"><br></div>
<br>

<div style="margin-left: 40px;">where <span style="font-style: italic;">&#952;</span> is the incidence angle and &#945;
    is set to 1.<br></div>
<br>&nbsp;&nbsp;&nbsp; For details of the CMOD5 model, the readers are referred to [1].<br><h4>Products Supported</h4>
<ul>
    <li>&nbsp; &nbsp;The
        operator now is only supported for ERS and ENVISAT (VV- and
        HH-polarized) products. The source product is assumed to have been calibrated
        before applying the operator.
    </li>
</ul>
<h4>Parameters Used</h4>&nbsp;&nbsp;&nbsp;The following parameters are used by the operator:
<ol>
    <li>Source Bands: All bands (real or virtual) of the source product.
        User can select one or more bands for producing multi-looked images. If
        no bands are selected, then by default all bands are selected.
    </li>
    <li>Window Size: The dimension of a window for which wind direction and speed are estimated.</li>
    </ol>
<div style="margin-left: 40px;"><img style="width: 600px; height: 500px;" alt=""
                                     src="images/wind_field_estimation_dlg.jpg">&nbsp;<br></div>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; Figure 1. Wind Field Estimation dialog box<br><h4>Visualize
    Estimated Wind Direction</h4>&nbsp;&nbsp;&nbsp;To&nbsp;view the estimated wind directions, the following steps
should be followed:<br>
<ol>
    <li>Bring up the image.</li>
    <li>Go to layer manager and add layer called "Wind Field Estimation Results".</li>
</ol>
&nbsp;&nbsp;&nbsp;Then
wind directions will be displayed as shown in the example below. Note
that the wind direction is indicated by double headed arrows because&nbsp;a 180�
ambiguity exists in the estimated wind direction. Also for those grids
in which land pixels are found, the wind directions are not estimated and
hence not displayed.<br><br>

<div style="margin-left: 40px;"><img style="width: 236px; height: 456px;" alt=""
                                     src="images/wind_field_estimation_example.jpg"><br></div>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; Figure 2. Example of wind direction display<br><h4>Wind Field
    Retrieval Result Report</h4>&nbsp;&nbsp;&nbsp;The
wind field estimation&nbsp;results are saved into an xml file
.s1tbx/log/wind_field_report.xml with the following information given
for each window in which wind estimation is made:<br>
<ol>
    <li><span style="font-style: italic;">lat</span>: Latitude of the central point in the window.</li>
    <li><span style="font-style: italic;">lon</span>: Longitude of&nbsp;the central point in the window.</li>
    <li><span style="font-style: italic;">speed</span>: Estimated wind speed in m/s.</li>
    <li><span style="font-style: italic;">dx</span>: X component of the estimated wind vector.</li>
    <li><span style="font-style: italic;">dy</span>: Y component of the estimated wind vector.</li>
    <li><span style="font-style: italic;">ratio</span>: In estimating wind direction, the spectrum of a given window is
        matched with a 2D polynomial (like f(x,y) = ax<sup>2</sup> + bxy + cy<sup>2</sup>
        + dx + ey +f). The ratio in the report is the ratio of the minor semi
        axes over the major semi axes of the 2D polynomial. Generally
        speaking,&nbsp;the smaller the ratio value, the more reliable the
        estimated wind direction.<br></li>
</ol>
<p style="font-style: italic;">Reference:</p>

<p>
    [1] H. Hersbach, CMOD5, &#8220;An Improved Geophysical Model Function for ERS
    C-Band Scatterometry&#8221;, Report of the European Centre Medium-Range
    Weather Forecasts (ECMWF), 2003.</p>

<p>[2] C. C. Wackerman, W. G.
    Pichel, P. Clemente-Colon, &#8220;Automated Estimation of Wind Vectors from
    SAR&#8221;, 12th Conference on Interactions of the Sea and Atmosphere, 2003.<br></p>

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